The present invention relates to a method for measurement through the use of a digital spectrum analyzer, according to which a test signal is supplied to a device under test and the output signal therefrom is converted into a digital signal and then subjected to fast Fourier transform, thereby measuring the spectral structure of the transfer function and the output signal of the device under test.
According to a conventional digital spectrum analyzer, as disclosed in, for example, U.S. Pat. No. 3,988,669 (issued on Oct. 26, 1976), a random noise is used as a test signal and the frequency range of measurement is measured at one time, or a sine-wave signal of a single frequency is generated, the frequency of which is sequentially changed to sweep the entire frequency range of measurement, and for each frequency change, a fast Fourier transform is carried out.
The measurement by the use of noise is short in time but poor in accuracy. In particular, a resonance or anti-resonance portion in the transfer function characteristic of the device under test, where the transfer function undergoes an abrupt change with frequency change, becomes dull, making accurate measurement impossible. On the other hand, in the case of sequentially changing the frequency of the single sine-wave signal, the portion of abrupt variations in the transfer function can also be measured faithfully, but this method is very time-consuming.
In the prior art, prior to the measurement, the input level of an AD converter for the input signal is set by selecting a suitable gain (or attenuation), which is commonly referred to as range setting (sensitivity setting), so that the input level of the AD converter assumes an optimum value for effectively utilizing its conversion range, and the measurement is performed at the thus set sensitivity over the entire frequency range of measurement. With this method, however, in the case where the level of a spectrum in the frequency range of measurement undergoes relatively great variations (that is, when the dynamic range is large), no accurate measurement can be achieved. That is, there are the possibilities that the high-level portion in the frequency range of measurement is saturated to produce a distortion and, in the low-level portion, that the number of effective bits in its converted digital signal output is small, resulting in lowered accuracy of measurement.
Moreover, according to the prior art, fast-Fourier-transformed line spectra are equally spaced in frequency over the entire frequency range of measurement. On account of this, measurement for the high-frequency region can be achieved with high resolution for frequency, but frequency resolution in the low-frequency region is low.